PostScript is a resolution-independent format. Fonts can be enlarged or reduced in size to accommodate any viewing resolution. When a viewer zooms in or out of a document, the text characters automatically scale accordingly. Thus it can be said that font characters are “scalable.”
Similarly, graphical objects based on vector graphics consisting of line segments and curves are also scalable. The line segments and curves can be enlarged or reduced in size by appropriately modifying the pixel coordinates of their control points.
Raster graphics, on the other hand, is not scalable. An image expressed in raster graphics is by its nature pixel resolution specific, and to enlarge or reduce the image involves digital image filtering and interpolation. Moreover, a raster image cannot be stretched beyond its original pixel resolution without introducing artificial data.